The cooling of hot solids downstream from a separator at the discharge end of, for example, an appended-bed reactor or, in general, a fluidized bed reactor, utilizing a fluidizing principle is known In general, processes and apparatus whereby at least part of the solids cooled are recycled to the fluidized bed reactor are known from DE-A-37 41 935, EP-A-0 407 730, and GB-A-2,148,734. By and large the earlier systems are not as versatile, efficient and controllable as may be required or desired.